The methanol-to-olefins (MTO) reaction is an industrially viable route for making the light olefins, ethylene and propylene, using feedstocks other than petroleum (e.g., natural gas, coal, and biomass). These feedstocks may be converted to syngas and then methanol as an intermediate. The MTO reaction can be carried out over solid acid catalysts such as microporous aluminosilicate (zeolites) and silicoaluminophosphate (SAPO) molecular sieves. The industrial catalyst for the MTO reaction is SAPO-34, a small-pore SAPO molecular sieve with the chabazite (CHA) framework topology that is currently utilized in commercial MTO plants in China. Depending upon reaction conditions, SAPO-34 can convert methanol to ethylene and propylene at 85-90% selectivity. The high selectivity toward light olefins is attributed to the material's optimal acidity (acid site strength and density) as well as the topology of the CHA framework, consisting of relatively large cavities (8.35 Å×8.35 Å×8.23 Å) that are accessible through eight-membered ring (8-MR) pore openings (3.8×3.8 Å). Only small linear molecules (alcohols and linear alkenes) can diffuse through the 8-MR pores, while larger branched and aromatic compounds, including methylated aromatic intermediates, remain trapped inside the cages. Despite its success, SAPO-34 suffers the shortcoming of requiring the use of an organic structure-directing agent (OSDA) to crystallize. Aluminosilicates (zeolites) also catalyze the reaction, but synthesizing them at high Si/Al ratios that are desirable for catalytic applications typically requires the use of OSDAs. The high cost and environmental concerns associated with removal of the OSDA from the materials prior to use has generated considerable interest in developing OSDA-free synthesis methods. Although the earliest synthetic zeolites were prepared in the absence of OSDAs, using only inorganic cations as the structure-directing species, they typically have high aluminum content (Si/Al<5) and thus limited uses, particularly in applications requiring solid acidity. CHA-type zeolites can be prepared in the absence of OSDAs, but their Si/Al ratios are too low to be of use in catalyzing reactions like MTO.
The present invention is directed to solving some of these shortcomings.